Hydroelectric turbine installations in which the turbine comprises several runner blades having an adjustable pitch are widely used. In these turbines, each runner blade (often simply called a "blade"), is pivotally connected to the hub having a longitudinal axis, the blades typically including a trunnion which is rotatable about an axis extending in a direction generally perpendicular to the hub. The rotation of each blade about its axis permits the turbine operator to vary the amount of power produced and seek the optimum efficiency of the hydroelectric installation under the entire range of operating conditions of the turbine.
In the hydroelectric industry, the most common type of turbine with adjustable pitch blades is referred to as a "Kaplan" turbine in which the axis of rotation of the blades is substantially perpendicular to the hub longitudinal axis. In relatively few instances where this condition is not met, the turbine is called a "Deriaz" turbine. However, to facilitate the reading of this application, in the following we will simply discuss the present invention in connection with Kaplan turbines because the principles of operation and operating parameters of Deriaz turbines that are of interest to the invention are substantially the same as those of Kaplan machines.
Kaplan turbines are also typically provided with adjustable wicket gates designed to regulate the flow of water admitted to the turbine. Accordingly, for each point of operation of such a turbine there is an optimum gate opening and blade opening condition that maximizes power output for the amount of flow passing through the turbine.
It is well recognized that hydroelectric power generation is generally socially more desirable than its counterparts which obtain energy from the combustion of fossil fuel or the fission or fusion of atoms. It is also widely accepted that Kaplan turbines materially improve the efficiency of hydroelectric installations. However, it is increasingly being suspected that certain Kaplan installations have various detrimental impacts on the environment, particularly on the fish population which is present in the water flowing through the turbine.
One of these potentially adverse impacts results from the very features of Kaplan turbines that increase the efficiency of hydroelectric installations using these turbines, namely the adjustable blades. Specifically, in a Kaplan turbine having its main axis generally parallel to the direction of the flow of water passing through the turbine, the pitch of the blades is adjustable from maximum to minimum blade opening or pitch, the blade forming a greater impediment to the flow of water when it is in the minimum pitch position (i.e., when the face of each blade is substantially perpendicular to the water flow).
Prior art Kaplan turbines are commonly provided with a frusto-spherical hub, i.e., in which the portion of the hub extending between two parallel planes passing through the intersection of the radiating lines R and the hub, is spherically-shaped as illustrated in FIGS. 2-5. In other words, and as more particularly shown in FIGS. 2 and 4, in such Kaplan turbines the surface region of the hub swept by the blades as the blades are moved between maximum and minimum pitch is not fully spherical. In that case, the blade inner surface conforms to the shape of the hub when the blade is at maximum pitch. However, gaps (often wedgeshaped) form between the blade inner surface and the hub surface as the blade departs from the maximum pitch position. A similar situation occurs in cases where the blade inner surface extends beyond the substantially spherical portion of the hub falling between the lines radiating from the hub center. Consequently, in both of these cases the surfaces of each blade facing the hub (i.e., the inner surface of each blade) do not fully conform to the outer surface of the hub over the entire range of blade positions. This means that as the blade departs from maximum pitch position (e.g., moving from position 5B to position 5A), a gap is formed between the hub and the blade edge, as more particularly illustrated in FIGS. 3 and 5.
Various studies have shown that gaps formed between the blades and the hub of a Kaplan turbine have several detrimental effects. First, such "detrimental" gaps (which are not to be confused with the functional clearances established between relatively movable part, such as for example clearance .delta. shown in FIG. 9A existing between the hub outer surface and the inner surface of the blade for suitable movement of the blades relative to the hub) formed between the hub and certain regions of the blades cause efficiency losses. This is because water leaking through such gaps typically lessens the ability of the blades to extract energy from the flow of water passing through the turbine. As can be readily appreciated, runner blades are configured so that water impinging thereon causes rotation of the runner to transform rotation of the runner into electrical energy. Water leaking through a gap therefore reduces the amount of water available to generate electrical energy, thereby reducing the efficiency of the turbine installation.
Furthermore, water leakage through a gap results in high turbulence and may also cause a phenomenon known as cavitation. As is well known in the art, cavitation occurs when components of the water flow move into regions of relatively low static pressures in the flow of water. Cavitation manifests itself by the production of bubbles of water vapor in low pressure regions of the water flow. When these bubbles of water vapor enter regions of higher pressure, they implode thereby causing damage (in the long run) to nearby structures such as the runner blades. As is well understood by those skilled in the art, a gap between the hub surface and the blade typically promotes cavitation. This is because the gap puts the high pressure side of the blade in fluid communication with its low pressure side (i.e., the suction side), potentially creating intense vortices which cause an undesirable cavitation condition.
In addition to efficiency losses and cavitation problems, gaps also form a trap for fish which are present in the water flowing through the turbine. It is believed that fish flowing into such gaps have a significantly greater chance of being injured or killed than fish flowing through other regions of the turbine. Recent efforts have therefore been undertaken to address the apparent propensity of Kaplan turbines to injure fish.
In particular, systems have been designed to divert fish away from Kaplan turbines. These systems include screens to keep fish out of the turbine, or structures configured to divert fish away from the turbine. It can be readily appreciated, however, that these prior art structures have several shortcomings. First, systems of the type necessitating separate structures consume some of the water normally flowing through the turbine thereby reducing the energy produced by the turbine installation. Second, it has been found that these systems are not fully effective to divert the entire fish population away from the turbine and may cause mortality to the fish. In addition, screens disturb the water flow and cause efficiency losses within the turbine. Finally, as can be readily appreciated, these additional structures, which in addition to not being entirely satisfactory, materially increase the cost of hydroelectric installations using Kaplan turbines.
Generally, various attempts have also been made to increase the efficiency of adjustable pitch propellers and turbines by reducing the gap formed in these mechanisms. For example, U.S. Pat. No. 2,498,072 issued Feb. 21, 1950 to Dean discloses an aircraft propeller in which the pitch of the blades is adjustable. To reduce air turbulence and drag in the region of the gap formed at the base of the blade, a seal made of molded rubber is attached to the hub embracing the blade airfoil.
More specifically, other attempts have been made to optimize the efficiency/cavitation ratio of Kaplan turbines and of hydroelectric turbines of other types. For example, U.S. Pat. No. 5,226,804 issued Jul. 13, 1993 to Do discloses a propeller-type runner in which the blades are fixed in position relative to the hub. The leading edge of each of the blades includes an enlarged forward region projecting toward the trailing edge of the immediately preceding blade. As noted in Do, it has been found that such a blade configuration reduces cavitation and produces superior torque.
Still another example of an approach used to improve the operating characteristics of certain rotating bladed implements is found in air fans, and in particular in axial flow fans having adjustable blades as disclosed in U.S. Pat. No. 2,382,535 issued on Aug. 14, 1945 to Bauer. In Bauer, to improve the efficiency of the fan, the fan is provided with a substantially spherically-shaped wheel periphery and a annular recess formed opposite the tip of the blades. The close tolerance between the wheel and the blades and the blades and the recess generally improves the efficiency of the fan.
The foregoing indicates that various attempts have been made to increase the efficiency of air propellers, fans, and Kaplan turbines. However, in view of the diverse detrimental effects resulting from the formation of gaps between the blades and hub or the blades and passageway of Kaplan turbine, it seems desirable to provide effective ways to reduce the size of these gaps and thereby improve certain operating characteristics of Kaplan turbines without materially impairing others.